The present invention relates to an apparatus for measuring the quantity of intake airflow passing through an intake passage of an engine, and more specifically to an airflow quantity measuring apparatus which is so constructed that, in electronically controlling the quantity of fuel injected in an engine, an intake airflow quantity measurement signal is produced so as to be effectively used in an electronic control device as one of detection signals for the operating conditions of the engine, and which can be maintained effectively.
In electronically controlling an engine, the operating conditions of the engine are continually monitored, and signals indicative of the operating conditions are detected. For example, injection quantity, ignition timing, etc., are computed on the basis of the detection signals. Based on the results of such operation, the engine control is executed.
Means for detecting the operating conditions of the engine include an engine speed detector, engine temperature sensor, exhaust gas temperature sensor, throttle opening detector and other detectors. An apparatus for measuring the quantity of intake airflow is one such means that operates in direct connection with the engine conditions.
An airflow quantity sensor of a heat-wire type is conventionally known as an example of the intake airflow quantity measuring apparatus. This sensor includes a temperature-sensitive element which is disposed in an intake passage and is adapted to generate heat when supplied with a heating current so that the temperature change of the temperature-sensitive element responsive to the quantity of airflow in the intake passage is monitored. The temperature-sensitive element is formed of a resistance element, e.g., platinum wire, whose resistance value varies with temperature. The heating current, which is supplied to the resistance element, is controlled so that the resistance element is kept at a predetermined temperature. In this case, the resistance element is disposed in the intake airflow, so that its heat radiation effect is influenced by the quantity of airflow. Therefore, the heating current needs to be supplied in proportion to the quantity of airflow in the intake passage. Thus, the intake airflow quantity can be measured by determining the voltage value and change of the heating current.
In the airflow quantity measuring apparatus of this heat-wire type, however, the temperature-sensitive element as a measuring element should be exposed, in use, to the airflow in the intake passage for a long period of time. Also, heated during use, the temperature-sensitive element catches dust in the air, so that its thermal conductivity varies with the amount of dust thereon. If the dust on the temperature-sensitive element increases, the heat radiation effect of the temperature-sensitive element ceases to be accurately responsive to the quantity of intake airflow, increasing measurement errors.
A measure to overcome these disadvantages is stated in Japanese Patent Disclosure No. 76182/79, in which dust on a temperature-sensitive element is burned off by heating the temperature-sensitive element to 800.degree. C. If subjected to such a high temperature for an extended time, e.g., 10 seconds or more, the temperature-sensitive element or its supporting members may possibly be damaged. In particular, the temperature-sensitive member, which is formed of a thin resistance wire, would be broken.